The present invention relates to a device for the under-stoichiometric gasification of spent liquor from chemical pulp production, which device comprises an upper reactor part, which upper reactor part is provided with an inlet having a nozzle for combusting the spent liquor, a separating part for separating a phase which is formed in the gasification and consists of solid and/or multimaterial from a phase which consists of combustible gaseous material, and also a product liquid space.
For a very long time, the commercially dominant process for recovering energy and chemicals from so-called black liquor, obtained when producing paper pulp in accordance with the kraft method, has conventionally been the so-called Tomlinson process, which uses a so-called recovery boiler.
The Swedish patent SE-C-448 173 describes a more modern process which is based on the understoichiometric gasification/pyrolysis (i.e. with a deficit of oxygen) of the black liquor in a reactor. In this context, the products are a phase consisting of solid and/or molten material, principally comprising sodium carbonate, sodium hydroxide and sodium sulphide, and an energy-rich, combustible gas phase, principally comprising carbon monoxide, carbon dioxide, methane, hydrogen and hydrogen sulphide. The mixed solid/molten phase and gas phase are cooled and are separated from each other in a separating part, which is connected to the reactor, by means of direct contact with green liquor, with the solid/molten phase being dissolved in the green liquor.
The green liquor is then conveyed to conventional causticization for producing white liquor. The gas phase is used as fuel for generating steam and/or electric power.
WO 95/35410 and WO 96/14468 are example of further developments of the process described in SE-C-448 173. These two patent applications deal, inter alia, with the problem with regard to being able to minimize the content of bicarbonate and carbonate in the liquor which is formed, with the solutions including minimizing the contact between the gas phase which is formed in the gasification and the smelt which is generated, and, respectively, returning hydrogen sulphide to the reactor in order to displace the reaction equilibrium in the reactor.
WO 97/37944 discloses a device for what is in the main complete oxidation of a concentrated liquid, which device possesses separate extractions for a gas phase which is formed in the reactor and for a smelt phase which is generated, respectively. In this context, the gas phase is extracted from the reactor in order to be washed and cooled separately using a cooling liquid, while the smelt phase falls down into a liquid bath.
In association with the understoichiometric gasification of spent liquor, WO 93/01890 discloses drawing off a flow of smelt phase mixed with gas phase, with the two phases then being separated from each other in a separate device in the form of a cyclone. The cyclone in this context possesses a tangential inlet for the two mixed phases and a liquid film on the inner side of the walls of the cyclone, with the smelt phase dissolving in the liquid film and then flowing out from a lower outlet. The gas phase is withdrawn from the cyclone by way of an upper outlet. WO 93/01890 suffers from the disadvantage that a centrifugal effect is not initiated in the reactor, thereby reducing the efficiency of the separation.
Even if the abovementioned prior art has to some degree contributed to enabling the generated green liquor to contain less bicarbonate, it has been found that additional measures can be required in certain cases in order to avoid generated green liquor containing bicarbonate and in order to minimize the content of carbonate in the green liquor as a consequence of the generated liquor absorbing carbon dioxide from the combustion gas.
The present invention provides a device for the understoichiometric gasification of spent liquor, which device decreases the abovementioned problems still further or eliminates them. The device is accordingly arranged to separate smelt phase generated on gasification from generated gas phase at a superheating temperature and with contact between the gas phase and the product liquid, which is formed when the smelt phase is dissolved in an aqueous liquid, being minimized. The minimized contact and the separation at a superheating temperature result in an optimized, high proportion of hydroxide ions being obtained in the green liquor/white liquor which is formed. The device is furthermore arranged to cool and protect exposed metal surfaces in the separating part without this markedly increasing the contact between the gas phase and the product liquid.
An aspect in one embodiment of the invention using a liquid bath which contacts the outer walls without contacting intermediate walls also achieves a possibility of regulating liquid levels in the separating part which is superior to that of the prior art. This is achieved at the same time as it is readily possible to apply and service measuring and levelling instruments from the outside.
The device according to the invention is defined in Patent Claim 1. According to the invention, the separating part accordingly comprises a first outlet for the phase consisting of solid and/or molten material, which outlet is arranged in connection with the lower part of the reactor part, in the form of a peripheral first outlet which principally follows the circumferential walls of the reactor part, which first outlet leads to the product liquid space, and a second outlet for the phase consisting of gaseous material, which outlet is arranged, in the main, concentrically within the said first outlet. In other words, the separating part possesses a centrally arranged second outlet for the gas phase, which second outlet is arranged such that, outside this outlet, and inside the circumferential walls of the reactor part, there is a first outlet for the phase consisting of solid and/or molten material (the smelt phase), which first outlet consequently encircles the second outlet and leads to the product liquid space.
One aspect of the invention uses the cyclone principle/centrifugal principle for separating the smelt phase and the gas phase from each other, with the device being designed to set the smelt phase and the gas phase in rotation in the reactor part, such that the heavier smelt phase is thrown against the circumferential walls of the reactor part and/or the separating part in order then to be conducted out of the reactor by way of the said first outlet. By contrast, the lighter gas phase is conducted away centrally by way of the said second outlet.
An important aspect of the invention is that rotation should be imparted to the spent liquor and the oxidizing agent in the reactor part as well. In this context, preference is given to the inlet and/or the combustion nozzle for the spent liquor being arranged, in the reactor part, to impart a rotating flow to the gasified spent liquor and the oxidizing agent, preferably by means of the said inlet/combustion nozzle being arranged with oblique spinnerets, or by the inlet/combustion nozzle being arranged to be rotating, or by the said inlet/combustion nozzle being arranged, in the main, tangentially with respect to the reactor part.
According to one aspect of the invention, the exposed metal parts/metal walls in the separating part are cooled and/or protected by being washed with liquid and/or jacketed with cooling liquid. The walls of the second outlet, i.e. for the gas phase, are expediently washed with liquid, both on the outside and on the inside, and/or jacketed with cooling liquid.
According to a preferred embodiment of the invention, the circumferential walls of the reactor part have, in the main, the same cross section, in the horizontal plane, along their principal vertical extent and further on down in a transition to the said separating part having the said first outlet, and also further on down into the lower part of the device. This gives the reactor part, and the device as a whole, an advantageous, in the main straight, shape. A considerable fraction of the reactions also takes place in the separating part, thereby providing an extended reaction space.
According to an alternative embodiment, the circumferential walls of the reactor part have a downwardly, tapering cross section, in the horizontal plane, along their principal vertical extent and further on down in a transition to the said separating part having the said first outlet. This amplifies the cyclone effect in a manner which is advantageous for the separation.
The second outlet, i.e. for the gas phase, can be arranged with an upwardly directed opening or with a downwardly directed opening, as described in more detail below in connection with the figures.
According to one aspect of the invention, the device accommodates both a product liquid space, i.e. a green liquor bath or a white liquor bath in which the smelt phase is dissolved, and a cooling liquid space, i.e. normally a condensate bath through which condensate bath the gas phase is caused to bubble, with the gas phase being washed, cooled and moisture-saturated, and a space for weak product liquid, i.e. a bath containing weak green liquor or weak white liquor in which a relatively small proportion of smelt phase, which is carried by the gas phase into the second outlet, is dissolved. In this connection, the different liquid spaces are preferably arranged at different levels in the device, with intermediate partitions being used for demarcation purposes.
According to one aspect of the invention, the reactor operates at a pressure of 1.5-150 bar (abs), preferably 1.5-50 bar; however, atmospheric pressure is also conceivable. The temperature in the reactor can be 500-1600xc2x0 C., preferably 900-1100xc2x0 C.